Sphingolipids serving as signaling molecules, have recently emerged as regulators of cell growth, differentiation, diverse cell phenotypes, and cell death. Signaling via sphingolipid turnover is exemplified by two distinct pathways: a) the formation of ceramide resulting from the activation of sphingomyelinase by TNF and a variety of other stimuli, and b) the formation of sphingosine-1-phosphate (S1P) upon activation of sphingosine kinase (SphK) by several growth factors such as platelet-derived growth factor and phorbol ester. Thus, it has been proposed that cells activate sphingomyelinase in response to cytokines, whereas growth factors activate SphK and thereby choose between the formation of ceramide that favors cell death versus S1P that inhibits death.
The importance of S1P and by consequence the importance of SphK is illustrated by a series of reports that showed their activity in endothelial cell survival and activation by cholesterol via the TNF pathway, in neutrophil activation, in transformed cell motility. (“Tumor necrosis factor-alpha-mediated signal transduction in human neutrophils: involvement of sphingomyelin metabolites in the priming effect of TNF-alpha on the fMLP-stimulated superoxide production.” Niwa et al., Life Sci 2000;66(3):245-56; Overexpression of SphK inhibits chemotactic motility of several transformed cell lines independently of cell surface receptors acting via its substrate, sphingosine-1-phosphate. Wang et al., “Sphingosine-1-phosphate inhibits motility of human breast cancer cells independently of cell surface receptors.”Cancer Res Dec. 15, 1999, 59(24):6185-91; “Activation of SphK by tumor necrosis factor-alpha inhibits apoptosis in human endothelial cells.” Xia et al., J Biol Chem Nov. 26, 1999;274(48):34499-505).
A mouse SphK is described in Kohama et al. (“Molecular cloning and functional characterization of murine SphK”. J Biol Chem. Sep 11, 1998;273(37):23722-8). However, the authors did not have conclusive data on where the mouse protein starts, even if they showed that the mouse cDNAs they obtained have SphK activity in vivo. Furthermore, no human sequence has been deposited for this enzyme in GenBank or Geneseq. In their paper, Kohama et al. have an alignment that shows only a part of a hypothetical human sequence.